FILE - This Nov. 10, 1987 file photo shows Treasury Secretary James Baker, left, and White House Chief of Staff Howard Baker huddle on Capitol Hill in Washington as they prepare to meet with Congressional negotiators to agree on ways to cut the federal deficit. (AP Photo/John Duricka, File)

WASHINGTON (AP) — Former Sen. Howard H. Baker Jr., a moderate Republican known as the politician who inquired what President Richard Nixon knew during the 1973 Senate Watergate hearings, has died. He was 88.

Baker, who served as U.S. ambassador to Japan from 2001 until early 2005, died Thursday at his home, according to an email distributed at the law firm where Baker was senior counsel. He died as a result of complications from a stroke suffered Saturday, the email said.

The scion of a political family, Baker served 18 years in the Senate, winning widespread respect from Republicans and Democrats alike and rising to the post of majority leader.

“Senator Baker truly earned his nickname: the Great Conciliator. I know he will be remembered with fondness by members of both political parties,” Sen. Mitch McConnell of Kentucky on the Senate floor Thursday, announcing Baker’s death to the chamber.

Baker was mentioned over the years as a possible vice presidential candidate, Supreme Court justice or CIA director. He rejected them all.

He ran once for president, in 1980, and thought about it several other times. And he served as White House chief of staff in the waning days of the Reagan administration.

But it was his instantly famous question — “What did the president know and when did he know it?” — that made him an enduring household name. It instantly focused the nation’s attention on the cover-up that perhaps more than the Watergate break-in itself eventually brought down Nixon’s presidency.

It came as he was serving as vice chairman, and thus leading Republican, on the Senate committee probing the June 1972 break-in at the Democratic headquarters and the cover-up by the Nixon administration.

Watergate, though it brought Baker national recognition, marked “the greatest disillusionment” of his political career, Baker said in a 1992 interview with The Associated Press.

“I believed that it was a political ploy of the Democrats, that it would come to nothing,” said Baker, who had seconded Richard Nixon’s nomination at the 1968 Republican convention. “But a few weeks into that, it began to dawn on me that there was more to it than I thought, and more to it than I liked.”

Baker, who served in a time when bipartisanship was more common in Washington, was Senate minority leader when he helped Democratic President Jimmy Carter win passage of the Panama Canal Treaty in 1978. Carter said at a 2011 symposium that Baker’s support for the divisive measure, which led to Panama regaining control of the passage, may have cost Baker the Republican nomination for president in 1980.

At the same symposium, Baker said he was still receiving letters every week from voters saying they would never forgive him for supporting the treaty.

“But that’s OK,” Baker said with a smile, “because I haven’t forgiven them either.”

He said he always considered his time as Senate majority leader, 1981 to 1985, the high point of his political career. He called it “the second-best job in town, only second to the presidency.”

He had been minority leader when the Republicans were swept into control of the chamber in the 1980 Reagan landslide and became the first Republican majority leader in decades.

Putting aside his own reservations about Reagan’s economic proposals, Baker played a key role in passage of legislation synonymous with the “Reagan Revolution” — major tax and spending cuts combined with a military buildup.

“Sometimes I’ve been angry and sometimes I’ve been happy and some days I’ve been frustrated,” Baker said in 1983 when he announced he would not seek another term in 1984. “But I can’t remember a single day when I’ve been bored.”

He left the Senate with an eye to another presidential bid in 1988, but instead returned to Washington in 1987 at Ronald Reagan’s request.

Reagan wanted Baker to replace ousted chief of staff Donald Regan amid the Iran-Contra scandal of arms-trading for hostages in Iran and diversion of profits to Nicaraguan rebels.

Baker recalled marshaling all his reasons for refusing the offer, but couldn’t turn down Reagan in the end. “I guess I am a pushover for presidents,” he said.

He and the Reagan White House weathered Iran-Contra. But Baker lost his last chance at the presidency.

“I have seen it up close and personal and I am convinced that I could do that job,” he said. “But that boat never came to dock.”

He left the White House in mid-1988 but remained involved both politically and diplomatically — traveling to Moscow in 1991 to meet with then-Soviet leader Mikhail Gorbachev before a summit meeting with President George H.W. Bush.

During much of the 1980s and ’90s, Baker had to grapple with the illness of his wife, Joy. She died in 1993 after an 11-year battle with cancer.

In 1996, Baker remarried, taking as his bride Kansas Sen. Nancy Landon Kassebaum, who was about to retire from the Senate after serving three terms. It was the first time two people who had served in the Senate had gotten married.

“Nancy and I have known each other for a long time. We share a lot of the same interests,” Baker said at the time. “And besides that, you know, love conquers a lot of things.”

During President George W. Bush’s first term, from 2001 to early 2005, Baker served as ambassador to Japan. His selection was in keeping with a tradition of selecting senior statesmen to represent the United States in Tokyo. His predecessors included Former House Speaker Tom Foley, Vice President Walter Mondale and Mike Mansfield, another former Senate majority leader.

More recently, Baker served as co-chair of a blue-ribbon commission on the national parks that in 2009 urged that Congress increase spending by at least $100 million per year to help improve and preserve the parks.

Baker was born in 1925 to a deeply political and staunchly Republican family in tiny Huntsville, Tenn. His neighbors called him “Howard Henry” or “Henry” to distinguish him from his father.

His father, Howard H. Baker Sr., served in the U.S. House from 1951 until he died in 1964. His stepmother, Irene Bailey Baker, won a special election and served the balance of his term. His grandmother Lillie Ladd Mauser once served as sheriff of Roane County, Tenn.

In 1951, Baker married Joy Dirksen, whose father, Everett Dirksen, was a U.S. senator from Illinois and Senate GOP leader for 10 years. Baker’s second wife also had a famous father: Alf Landon, onetime Kansas governor and 1936 GOP presidential nominee.

Baker received a law degree from the University of Tennessee in 1949 and was a partner in a firm with offices in Knoxville and Washington. He also served on several corporate boards.

In 2008, the University of Tennessee opened its Howard Baker Jr. Center for Public Policy, which includes a museum and study center focusing on how the government grapples with issues amid the rough and tumble of party politics. It houses the papers and archives of dozens of political leaders, including Baker.

Baker and his first wife had two children: Darek, born in 1953, and Cynthia, known as “Cissy,” born in 1956.

Baker, an accomplished amateur photographer, was known to carry a camera with him wherever he went. But he didn’t take any photos during the Watergate hearings.

“I felt that it was beneath the dignity of the event,” he said years later. “It turned out the event had no dignity and I should have taken pictures.”